[PATCH][libcxx] Prevent '\b' from backing up into invalid memory (case 2)

[William Fisher]

This patch fixes a less likely case where '\b' can back up into invalid
memory, when driven by a regex_iterator (for case 1, see r185273 or
http://llvm.org/bugs/show_bug.cgi?id=16240)

The attached test program also supplies a test for the case 1 fix in
r185273.

Thanks,
-Bill
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